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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Magnetic properties of Ni nanoparticles embedded in silica matrix (KIT-6) synthesized via novel chemical route

Thermally stable Ni nanoparticles have been embedded in mesoporous silica matrix (KIT-6) via novel chemical reduction method by using superhydride as reducing agent. X-ray diffraction (XRD) study confirms that pure and embedded Ni nanoparticles crystallize in face centered cubic (fcc) structure. Crystallite sizes of pure Ni, 4 wt% and 8 wt% Ni in silica were estimated to be 6.0 nm, 10.4 nm and 10.5 nm, respectively. Morphology and dispersion of Ni in silica matrix were studied by scanning electron microscopy (SEM). Magnetic study shows enhancement of magnetic moments of Ni nanoparticles embedded in silica matrix compared with that of pure Ni. The result has been interpreted on the basis of size reduction and magnetic exchange effects. Saturation magnetization values for pure Ni, 4 wt% and 8 wt% Ni in silica were found to be 15.77 emu/g, 5.08 emu/g and 2.00 emu/g whereas coercivity values were 33.72 Oe, 92.47 Oe and 64.70 Oe, respectively. We anticipate that the observed magnetic properties may find application as soft magnetic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Influence of fuel on phase formation of ZnFe{sub 2}O{sub 4} prepared by self-propagated combustion route

Zinc iron oxide (ZnFe{sub 2}O{sub 4}) nanoparticles were prepared by self-propagated combustion route.The fuel plays a major role on the formation of structure and particle size. Here three different fuels like alanine, glycine and proline were used to synthesis the zinc iron oxide nanoparticle. Influence of combustion nature through the fuel, the phase formation, particle size, band gap and surface morphology has been modified. The prepared powder was characterized by powder X-ray diffraction method (PXRD), field-emission scanning electron microscopy (FE-SEM) and the composition of the material was analysed by X-ray energy dispersive spectroscopy (EDAX).The elemental mapping was confirmed the uniform distribution of Zn, Fe and O elements in the prepared material of ZnFe{sub 2}O{sub 4}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Towards a predictive route for selection of doping elements for the thermoelectric compound PbTe from first-principles

Striving for improvements of the thermoelectric (TE) properties of the technologically important lead telluride (PbTe) compound, we investigate the influence of different doping elements on the thermal conductivity, Seebeck coefficient, and electrical conductivity applying density functional theory calculations. Our approach combines total-energy calculations yielding lattice vibrational properties with the Boltzmann transport theory to obtain electronic transport properties. We find that doping with elements from the 1st and 3rd columns of the periodic table reduces the sound velocity and, consequently, the lattice thermal conductivity, while 2nd column dopants have no such influence. Furthermore, 1.6 at. % doping with 4th and 5th column elements provides the highest reduction of lattice thermal conductivity. Out of this group, Hf doping results in maximum reduction of the sound velocity from 2030 m s{sup −1} for pure PbTe to 1370 m s{sup −1}, which is equivalent to ca. 32% reduction of lattice thermal conductivity. The highest power factor values calculated for 1.6 at. % doping range between 40 and 56 μW cm{sup −1} K{sup −2}, and are obtained for substitution with dopants having the same valence as Pb or Te, such as those located at the 2nd, 14th, and 16th columns of the periodic table. We demonstrate how this method may be generalized for dopant-selection-oriented materials design aimed at improving TE performance of other compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Oxide 2D electron gases as a route for high carrier densities on (001) Si

Two dimensional electron gases (2DEGs) formed at the interfaces of oxide heterostructures draw considerable interest owing to their unique physics and potential applications. Growing such heterostructures on conventional semiconductors has the potential to integrate their functionality with semiconductor device technology. We demonstrate 2DEGs on a conventional semiconductor by growing GdTiO{sub 3}-SrTiO{sub 3} on silicon. Structural analysis confirms the epitaxial growth of heterostructures with abrupt interfaces and a high degree of crystallinity. Transport measurements show the conduction to be an interface effect, ∼9 × 10{sup 13} cm{sup −2} electrons per interface. Good agreement is demonstrated between the electronic behavior of structures grown on Si and on an oxide substrate, validating the robustness of this approach to bridge between lab-scale samples to a scalable, technologically relevant materials system.

36 MATERIALS SCIENCE↗
DOE OSTI2015

A direct route for the investigation of crystalline field effects on the spectral properties of Eu{sup 3+} ions in designed nanostructures

The spectral properties of lanthanide ions doped in a solid-state matrix, especially the Stark splitting and emission shifts, are strongly influenced by the local crystalline field. Here, we use a direct approach to quantitatively investigate the crystalline field effect on these spectral properties of Eu{sup 3+} ions in our designed β-PbF{sub 2}:Eu{sup 3+} and BaF{sub 2}:Eu{sup 3+} nanostructures. The extremely similar structures in which the central Eu{sup 3+} ions have the same site symmetry allow the local crystalline field to be simplified as the same point charge electrostatic field model. In this model, the direction and intensity of the electrostatic field are related to the distortion and expansion degree of the charge cloud of the luminescent center, respectively, and further determine the Stark splitting and emission shift. The theoretical analysis and charge-cloud stimulations were in good agreement with the experimental results. The direction and intensity of the crystalline field were calculated, and showed that the emission spectra shift to red with the increase in intensity. This work provides a comprehensive understanding of the spectral changes induced by a crystalline field, which is of great significance for the design of materials with the desired spectral properties.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Optical and electrical properties of undoped and boron doped zinc oxide synthesized by chemical route

We have synthesized and studied the boron doped ZnO nanostructure thin films. The crystallinity of undoped and boron (B) doped ZnO (BZO) has been studied from XRD results. Using the Debye-Scherrer Formula, the grain size has been evaluated, which was found to decrease with increased doping concentration. The optical and electrical properties of (1, 3, 5 wt%) B-doped ZnO (BZO) has been investigated with reference to the undoped counterpart. The UV-VIS spectroscopic analysis revealed that the transmittance for undoped ZnO is maximum and it decreases with doping up to 3% but increases for 5% BZO. The dark as well as photo current–voltage (I–V) characteristics have been investigated in details and the changes occurred in the I-V characteristics with doping concentration as well as under illumination are also quite significant.

36 MATERIALS SCIENCE↗
DOE OSTI2016

First-principles calculations of thermoelectric properties of TiN/MgO superlattices: The route for an enhancement of thermoelectric effects in artificial nanostructures

We present the thermoelectric properties of TiN/MgO superlattices employing first-principles calculation techniques. The Seebeck coefficients, the electrical conductances, the thermal conductances, and the figure of merit are investigated employing electrical and thermal transport calculations based on density functional theory combined with the nonequilibrium Green's function and nonequilibrium molecular dynamics simulation methods. The TiN/MgO superlattices with a small lattice mismatch at the interfaces are ideal systems to study the way for an enhancement of thermoelectric properties in artificial nanostructures. We find that the interfacial scattering between the two materials in the metal/insulator superlattices causes the electrical conductance to change rapidly, which enhances the Seebeck coefficient significantly. We show that the figure of merit for the artificial superlattice nanostructures has a much larger value compared with that of the bulk material and changes drastically with the superlattice configurations at the atomistic level.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Dark and photo conductivity analysis of Cu doped CdSe-PVA nanocomposites synthesized by chemical route

Present communication deals with the study of electrical conductivity measurements of Cu doped CdSe-PVA nanocomposite via chemical method. In electrical measurements, the dark conductivity (σ{sub d}) and the photoconductivity (σ{sub ph}) of CdSe prepared thin films have been studied in the temperature range of 308–343 K. The effect of temperature and the intensity on conductivity has been analyzed for CdSe and CdSe:Cu nanocomposite films. The conductivity of all the samples increases with increasing temperature indicating the semiconducting behavior of the samples. The value of photo activation energy is less than the dark activation energy due to the shift in energy levels under illumination.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Synthesis and characteristic of nanocrystalline La{sub 0.7}Sr{sub 0.3}MnO{sub 3} manganites by solid state reaction route

Nanocrystalline stoichiometric La{sub 0.7}Sr{sub 0.3}MnO{sub 3} (x=0.3) manganites have been synthesized through solid-state reaction by ball milling mechanical method at two different sintering temperatures 1250°C and 1350°C. The synthesized samples were characterized using X-ray diffraction (XRD) and found to have rhombohedral crystal structure (R-3c). The calcined samples exhibited a pure single phase perovskite, had a crystallite size of about 47-51 nm. The morphology of the prepared nanocrystalline manganites were recorded by the field emission gun-scanning electron microscope (FEG-SEM) and EDAX.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Urchin-like artificial gallium oxide nanowires grown by a novel MOCVD/CVD-based route for random laser application

A novel procedure based on a two-step method was developed to obtain β-Ga{sub 2}O{sub 3} nanowires by the chemical vapor deposition (CVD) method. The first step consists in the gallium micro-spheres growth inside a metal-organic chemical vapor deposition environment, using an organometallic precursor. Nanoscale spheres covering the microspheres were obtained. The second step involves the CVD oxidization of the gallium micro-spheres, which allow the formation of β-Ga{sub 2}O{sub 3} nanowires on the micro-sphere surface, with the final result being a nanostructure mimicking nature's sea urchin morphology. The grown nanomaterial is characterized by several techniques, including X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray, transmission electron microscopy, and photoluminescence. A discussion about the growth mechanism and the optical properties of the β-Ga{sub 2}O{sub 3} material is presented considering its unknown true bandgap value (extending from 4.4 to 5.68 eV). As an application, the scattering properties of the nanomaterial are exploited to demonstrate random laser emission (around 570 nm) when it is permeated with a laser dye liquid solution.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Synthesis and characterization of CdSe quantum dots dispersed in PVA matrix by chemical route

CdSe quantum dots using polyvinyl alcohol as a capping agent have been synthesized via a simple heat induced thermolysis technique. The structural analysis of CdSe/PVA thin film was studied by X-ray diffraction, which confirms crystalline nature of the prepared film. The surface morphology and particle size of the prepared sample was studied by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The SEM studies of CdSe/PVA thin film shows the average size of particles in the form of clusters of several quantum dots in the range of 10-20 nm. The morphology of CdSe/PVA thin film was further examined by TEM. The TEM image shows the fringes of tiny dots with average sizes of 4-7 nm. The optical properties of CdSe/PVA thin film were studied by UV-VIS absorption spectroscopy. The CdSe/PVA quantum dots follow the role of direct transition and the optical band gap is found to be 4.03 eV. From dc conductivity measurement, the observed value of activation energy was found to be 0.71 eV.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Synthesis of Cu{sub 2}ZnSnS{sub 4} nanoparticles by solvothermal route

CZTS nanoparticles of size ~ 20 nm have been synthesized by solvothermal method. X-ray diffraction pattern confirms the growth of single phase CZTS nanoparticles with kesterite structure and lattice parameter obtained as a = b = 0.54 nm and c = 1.08 nm. The Raman peak observed at 338 cm{sup −1} also confirms the formation of pure CZTS phase. High resolution transition electron microscopy image confirms the good crystallinity of the CZTS nanoparticles with the growth along (112) plane. Energy dispersive spectroscopy confirms the stoichiometry of the CZTS nanoparticles. The CZTS nanoparticles exhibits a strong optical absorption in the visible region with direct band gap ~1.5 eV, which is optimum for photovoltaic application.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Magnetic properties of CoFe{sub 2}O{sub 4} and ZnFe{sub 2}O{sub 4} nanoparticles synthesized by novel chemical route

CoFe{sub 2}O{sub 4} and ZnFe{sub 2}O{sub 4} nanoparticles were synthesized by modified and cost effective sol-gel autocombustion method. X-ray diffraction study confirms the spinel phase formation of face centered cubic lattice with space group Fd3m and without any impurity. Magnetic measurements demonstrate that the CoFe{sub 2}O{sub 4} shows saturation magnetization of 2.73 µ{sub B}/f.u. where as ZnFe{sub 2}O{sub 4} shows paramagnetic behavior. Magnetic behavior of CoFe{sub 2}O{sub 4} and ZnFe{sub 2}O{sub 4} is confirmed from Mössbauer studies. Effect of sintering on structural, magnetic and cation occupancy of substituted cobalt ferrite is discussed in this paper.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Selective heteroepitaxy on deeply grooved substrate: A route to low cost semipolar GaN platforms of bulk quality

Semipolar GaN crystal stripes larger than 100 μm with dislocation densities below 5 × 10{sup 6} cm{sup −2} are achieved using a low cost fabrication process. An original sapphire patterning procedure is proposed, enabling selective growth of semipolar oriented GaN stripes while confining the defects to specific areas. Radiative and non-radiative crystalline defects are investigated by cathodoluminescence and can be correlated to the development of crystal microstructure during the growth process. A dislocation reduction mechanism, supported by transmission electron microscopy, is proposed. This method represents a step forward toward low-cost quasi-bulk semipolar GaN epitaxial platforms with an excellent structural quality which will allow for even more efficient III-nitride based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Temperature dependence of secondary electron emission: A new route to nanoscale temperature measurement using scanning electron microscopy

Scanning electron microscopy (SEM) is ubiquitous for imaging but is not generally regarded as a tool for thermal measurements. Here, the temperature dependence of secondary electron (SE) emission from a sample's surface is investigated. Spatially uniform SEM images and the net charge flowing through a sample were recorded at different temperatures to quantify the temperature dependence of SE emission and electron absorption. The measurements also demonstrated charge conservation during thermal cycling by placing the sample inside a Faraday cup to capture the emitted SEs and back-scattered electrons from the sample. The temperature dependence of SE emission was measured for four semiconducting materials (Si, GaP, InP, and GaAs) with response coefficients found to be of magnitudes ~100-1000 ppm/K. The detection limits for temperature changes were no more than ±8 °C for 60 s acquisition time.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Confinement of relativistic electrons in a magnetic mirror en route to a magnetized relativistic pair plasma

Creating a magnetized relativistic pair plasma in the laboratory would enable the exploration of unique plasma physics relevant to some of the most energetic events in the universe. As a step toward a laboratory pair plasma, we have demonstrated an effective confinement of multi-MeV electrons inside a pulsed-power-driven 13 T magnetic mirror field with a mirror ratio of 2.6. The confinement is diagnosed by measuring the axial and radial losses with magnetic spectrometers. The loss spectra are consistent with ≤2.5 MeV electrons confined in the mirror for ∼1 ns. With a source of 1012 electron-positron pairs at comparable energies, this magnetic mirror would confine a relativistic pair plasma with Lorentz factor γ∼6 and magnetization σ∼40.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗